Most exoplanets that we thought were wet are probably dry inside

For years, some exoplanets looked like good bets to have water, some with deep global oceans. They sit between Earth and Neptune in size, and several early spectra hinted at water friendly chemistry close to home, about 124 light years away.

A new study argues that most of these worlds are not covered in water. It shows that water gets pulled into the planet’s depths during formation, leaving only a thin surface budget and no global sea.

Lead author Aaron Werlen from ETH Zurich, led the analysis with collaborators at the Max Planck Institute for Astronomy (MPIA) and UCLA.

Their work focuses on sub-Neptune planets, which are larger than Earth and smaller than Neptune, a class missing from our solar system but common in the galaxy.

The authors set K2-18b in context without turning the story into a single-planet tale. They test a population of planets to ask a simple question with hard chemistry behind it.

Exoplanets and atmospheric water

The team computed how a young planet’s hot atmosphere interacts with its molten interior. Earlier equilibrium work showed that gases do not float above magma untouched, they trade atoms and transform.

Across their synthetic population, the final water inventory stayed below about 1.5 percent by mass. That falls far short of a world with a global ocean.

“Our calculations show that this scenario is not possible,” said Caroline Dorn, professor of exoplanets at ETH Zurich.

Why exoplanet water sinks instead of stays

Early on, many sub-Neptunes wear hydrogen rich envelopes and host a deep magma ocean.

The hot gas and the liquid rock meet for millions of years, and the chemistry favors moving hydrogen and oxygen into metal rich compounds that sink.

“Water on planets is much more limited than previously believed,” said Dorn. That chemical pull hides water in the interior and leaves the atmosphere hydrogen heavy. 

Where Hycean hopes came from

The idea of Hycean planets suggested thick hydrogen atmospheres sitting over deep global oceans, widening the range of potentially habitable worlds.

It was an innovative target list, and it drew attention because those atmospheres are easier to probe than thin Earth like ones.

K2-18b became a poster child after a preprint reported methane and carbon dioxide in its atmosphere. Those gases can be compatible with oceans, but chemistry alone never proves an ocean exists.

Under the new framework, sub-Neptunes like K2-18b are unlikely to carry planet wide oceans near the surface. Their water budget is modest, and much of it is trapped inside.

Even when the atmosphere looks water rich, it can be the product of a small envelope where a little water goes a long way by percentage. The bulk reservoir still fails to add up to an ocean.

Chemistry, water, and exoplanets

At the pressures and temperatures near the base of these atmospheres, hydrogen and water likely mix into one phase rather than separate into layers.

High pressure research indicates that H2 and H2O are often fully miscible under sub-Neptune conditions.

That means a clean water layer above a hydrogen layer is unstable at depth. The fluid behaves more like a single, super hot mix than a stratified stack.

A snow line is the distance in a young disk where water freezes to ice. Planets formed outside it pick up more ice and hydrogen, while those assembled inside it pick up less.

The model suggests that planets built inside the snow line, where less hydrogen is available, can end up with envelopes dominated by water as a fraction.

That does not require large absolute amounts of water, only small, hydrogen poor atmospheres.

Planets built beyond the snow line grow hydrogen rich envelopes, then dilute any water and bury much of their oxygen and hydrogen into the metallic interior. The end result is a low apparent water budget.

Why Earth looks ordinary after all

The new picture puts Earth’s water into a common range rather than on an outlier pedestal. Estimates of interior water suggest up to several ocean masses may be stored in the mantle.

If many sub-Neptunes equilibrate to similar or lower total water fractions, then Earth’s combined surface plus interior inventory looks normal rather than special.

Spectra can reveal methane, carbon dioxide, and water vapor. They cannot, on their own, certify a surface ocean under a hydrogen envelope.

Future observatories may separate these cases by pinning down temperatures, pressures, and mean molecular weights with higher precision. For now, chemistry driven interior coupling remains a crucial piece of the puzzle.

“These findings challenge the classic link between ice rich formation and water rich atmospheres,” concluded Werlen.

The study is published in The Astrophysical Journal Letters.

Image credits: ESA/Hubble.

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